Processing bodies accumulate in human cytomegalovirus-infected cells and do not affect viral replication at high multiplicity of infection

Strategies for Success. Viral Infections and Membraneless Organelles

Regulation of RNA homeostasis or “RNAstasis” is a central step in eukaryotic gene expression. From transcription to decay, cellular messenger RNAs (mRNAs) associate with specific proteins in order to regulate their entire cycle, including mRNA localization, translation and degradation, among others. The best characterized of such RNA-protein complexes, today named membraneless organelles, are Stress Granules (SGs) and Processing Bodies (PBs) which are involved in RNA storage and RNA decay/storage, respectively. Given that SGs and PBs are generally associated with repression of gene expression, viruses have evolved different mechanisms to counteract their assembly or to use them in their favor to successfully replicate within the host environment. In this review we summarize the current knowledge about the viral regulation of SGs and PBs, which could be a potential novel target for the development of broad-spectrum antiviral therapies.

RNA processing bodies are disassembled during Old World alphavirus infection

RNA processing bodies (P-bodies) are non-membranous cytoplasmic aggregates of mRNA and proteins involved in mRNA decay and translation repression. P-bodies actively respond to environmental stresses, associated with another type of RNA granules, known as stress granules (SGs). Alphaviruses were previously shown to block SG induction at late stages of infection, which is important for efficient viral growth. In this study, we found that P-bodies were disassembled or reduced in number very early in infection with Semliki Forest virus (SFV) or chikungunya virus (CHIKV) in a panel of cell lines. Similar to SGs, reinduction of P-bodies by a second stress (sodium arsenite) was also blocked in infected cells. The disassembly of P-bodies still occurred in non-phosphorylatable eIF2α mouse embryonal fibroblasts (MEFs) that are impaired in SG assembly. Studies of translation status by ribopuromycylation showed that P-body disassembly is independent of host translation shutoff, which requires the phosphorylation of eIF2α in the SFV- or CHIKV-infected cells. Labelling of newly synthesized RNA with bromo-UTP showed that host transcription shutoff correlated with P-body disassembly at the same early stage (3–4 h) after infection. However, inhibition of global transcription with actinomycin D (ActD) failed to disassemble P-bodies as effectively as the viruses did. Interestingly, blocking nuclear import with importazole led to an efficient P-bodies loss. Our data reveal that P-bodies are disassembled independently from SG formation at early stages of Old World alphavirus infection and that nuclear import is involved in the dynamic of P-bodies.

Viral Regulation of RNA Granules in Infected Cells

RNA granules are cytoplasmic, microscopically visible, non-membrane ribo-nucleoprotein structures and are important posttranscriptional regulators in gene expression by controlling RNA translation and stability. TIA/G3BP/PABP-specific stress granules (SG) and GW182/DCP-specific RNA processing bodies (PB) are two major distinguishable RNA granules in somatic cells and contain various ribosomal subunits, translation factors, scaffold proteins, RNA-binding proteins, RNA decay enzymes and helicases to exclude mRNAs from the cellular active translational pool. Although SG formation is inducible due to cellular stress, PB exist physiologically in every cell. Both RNA granules are important components of the host antiviral defense. Virus infection imposes stress on host cells and thus induces SG formation. However, both RNA and DNA viruses must confront the hostile environment of host innate immunity and apply various strategies to block the formation of SG and PB for their effective infection and multiplication. This review summarizes the current research development in the field and the mechanisms of how individual viruses suppress the formation of host SG and PB for virus production.

Who Regulates Whom? An Overview of RNA Granules and Viral Infections

After viral infection, host cells respond by mounting an anti-viral stress response in order to create a hostile atmosphere for viral replication, leading to the shut-off of mRNA translation (protein synthesis) and the assembly of RNA granules. Two of these RNA granules have been well characterized in yeast and mammalian cells, stress granules (SGs), which are translationally silent sites of RNA triage and processing bodies (PBs), which are involved in mRNA degradation. This review discusses the role of these RNA granules in the evasion of anti-viral stress responses through virus-induced remodeling of cellular ribonucleoproteins (RNPs).

Human Cytomegalovirus pTRS1 and pIRS1 Antagonize Protein Kinase R To Facilitate Virus Replication

Human cytomegalovirus (HCMV) counteracts host defenses that otherwise act to limit viral protein synthesis. One such defense is the antiviral kinase protein kinase R (PKR), which inactivates the eukaryotic initiation factor 2 (eIF2) translation initiation factor upon binding to viral double-stranded RNAs. Previously, the viral TRS1 and IRS1 proteins were found to antagonize the antiviral kinase PKR outside the context of HCMV infection, and the expression of either pTRS1 or pIRS1 was shown to be necessary for HCMV replication. In this study, we found that expression of either pTRS1 or pIRS1 is necessary to prevent PKR activation during HCMV infection and that antagonism of PKR is critical for efficient viral replication. Consistent with a previous study, we observed decreased overall levels of protein synthesis, reduced viral protein expression, and diminished virus replication in the absence of both pTRS1 and pIRS1. In addition, both PKR and eIF2α were phosphorylated during infection when pTRS1 and pIRS1 were absent. We also found that expression of pTRS1 was both necessary and sufficient to prevent stress granule formation in response to eIF2α phosphorylation. Depletion of PKR prevented eIF2α phosphorylation, rescued HCMV replication and protein synthesis, and reversed the accumulation of stress granules in infected cells. Infection with an HCMV mutant lacking the pTRS1 PKR binding domain resulted in PKR activation, suggesting that pTRS1 inhibits PKR through a direct interaction. Together our results show that antagonism of PKR by HCMV pTRS1 and pIRS1 is critical for viral protein expression and efficient HCMV replication.

IMPORTANCE

To successfully replicate, viruses must counteract host defenses that limit viral protein synthesis. We have identified inhibition of the antiviral kinase PKR by the viral proteins TRS1 and IRS1 and shown that this is a critical step in HCMV replication. Our results suggest that inhibiting pTRS1 and pIRS1 function or restoring PKR activity during infection may be a successful strategy to limit HCMV disease.

Analysis of human cytomegalovirus replication in primary cultured human corneal endothelial cells

Background/aims

Since the first case of human cytomegalovirus (HCMV)-induced corneal endotheliitis in which HCMV DNA was detected from the patient's aqueous humour using PCR, the clinical evidence for HCMV endotheliitis has been accumulating. However, it remains to be confirmed whether HCMV can efficiently replicate in corneal endothelial cells. We, therefore, sought to determine whether primary cultured human corneal endothelial cells (HCECs) could support HCMV replication.

Methods

Human foreskin fibroblasts (HFFs) have been shown to be fully permissive for HCMV replication, and are commonly used as an in vitro model for HCMV lytic replication. Therefore, primary cultured HCECs or HFFs were infected with the vascular endotheliotropic HCMV strain TB40/E or laboratory strain Towne. We then compared viral mRNA and protein expression, genome replication and growth between the TB40/E-infected and Towne-infected HCECs and HFFs.

Results

When HCECs were infected with TB40/E or Towne, rounded cells resembling owl's eyes as well as viral antigens were detected. Viral mRNA synthesis and protein expression proceeded efficiently in the HCECs and HFFs infected with TB40/E or Towne at a high multiplicity of infection (MOI). Similarly, the viral genome was also effectively replicated, with UL44—a viral DNA polymerase processivity factor—foci observed in the nuclei of HCECs. HCECs produced a substantial number of infectious virions after infection with TB40/E at both a high and low MOI.

Conclusions

Primary cultured HCECs could efficiently support HCMV replication after infection at both a high and low MOI.

Viral activation of MK2-hsp27-p115RhoGEF-RhoA signaling axis causes cytoskeletal rearrangements, p-body disruption and ARE-mRNA stabilization

Kaposi's sarcoma-associated herpesvirus (KSHV) is the infectious cause of several AIDS-related cancers, including the endothelial cell (EC) neoplasm Kaposi's sarcoma (KS). KSHV-infected ECs secrete abundant host-derived pro-inflammatory molecules and angiogenic factors that contribute to tumorigenesis. The precise contributions of viral gene products to this secretory phenotype remain to be elucidated, but there is emerging evidence for post-transcriptional regulation. The Kaposin B (KapB) protein is thought to contribute to the secretory phenotype in infected cells by binding and activating the stress-responsive kinase MK2, thereby selectively blocking decay of AU-rich mRNAs (ARE-mRNAs) encoding pro-inflammatory cytokines and angiogenic factors. Processing bodies (PBs) are cytoplasmic ribonucleoprotein foci in which ARE-mRNAs normally undergo rapid 5′ to 3′ decay. Here, we demonstrate that PB dispersion is a feature of latent KSHV infection, which is dependent on kaposin protein expression. KapB is sufficient to disperse PBs, and KapB-mediated ARE-mRNA stabilization could be partially reversed by treatments that restore PBs. Using a combination of genetic and chemical approaches we provide evidence that KapB-mediated PB dispersion is dependent on activation of a non-canonical Rho-GTPase signaling axis involving MK2, hsp27, p115RhoGEF and RhoA. PB dispersion in latently infected cells is likewise dependent on p115RhoGEF. In addition to PB dispersion, KapB-mediated RhoA activation in primary ECs caused actin stress fiber formation, increased cell motility and angiogenesis; these effects were dependent on the activity of the RhoA substrate kinases ROCK1/2. By contrast, KapB-mediated PB dispersion occurred in a ROCK1/2-independent manner. Taken together, these observations position KapB as a key contributor to viral reprogramming of ECs, capable of eliciting many of the phenotypes characteristic of KS tumor cells, and strongly contributing to the post-transcriptional control of EC gene expression and secretion.

We have only scratched the surface in understanding how viruses control host gene expression. Several viruses disrupt important sites of post-transcriptional control of gene expression known as processing bodies (PBs), but underlying regulatory mechanisms and biological relevance remain poorly understood in most cases. Our study shows that the Kaposin B (KapB) protein of Kaposi's sarcoma (KS)-associated herpesvirus, known to block the degradation of a class of labile host mRNAs, does so by constitutively activating a signaling axis involving MK2, hsp27, p115RhoGEF and RhoA, thereby dispersing PBs. Thus, PB disruption may support the secretion of host pro-inflammatory cytokines and angiogenic factors that underlies KS tumor formation. Furthermore, by activating RhoA, KapB also causes cytoskeletal rearrangements, accelerated cell migration and angiogenesis in an endothelial cell model. Our findings position KapB as a key contributor to viral reprogramming of endothelial cells.